n, k = map(int, input().split())
S = [ input() for i in range(n) ]

A = [ S[i::k] for i in range(k) ]
for i in range(k):
	A[i] = [*reversed(sorted(A[i]))]

T = []
while len(T) < n:
	for i in range(k):
		if A[i]:
			T.append(A[i].pop())


print("Yes" if T == [*sorted(T)] else "No")